1. Field of the Invention
The invention relates generally to circuitry that may be used to perform a NOR logic function. More particularly, the invention relates to half current switch emitter follower ("HCSEF") circuitry that performs a NOR function and has bias levels compatible with current switch emitter follower ("CSEF") circuitry and that utilizes less power than CSEF circuits. The novel circuit is referred to as a "NOR.sub.i " circuit since it utilizes a transistor operated in the inverse active mode as its current source.
The invention also relates to a companion reference bias generator for the NOR.sub.i circuit. The reference bias generator compensates for process, power supply, and temperature changes in order to maintain a constant down level (or lower logic level) in the NOR.sub.i circuit.
2. Description of the Related Art
Half current switch circuits that perform a logical NOR function are well known. For example, one such circuit is described in an article entitled "Half-Current Switch Circuit" by Bass, et al, published in December, 1980, in IBM Technical Disclosure Bulletin, Volume 23, No. 7A ("IBM" is a trademark owned by the International Business Machines Corporation).
The advantage of using HCSEF circuits on integrated circuit also containing CSEF circuitry is the potential for overall circuit performance improvement (in particular, power and density savings) when only the NOR logic function is required. However, a compatibility problem exists when using HCSEF circuits on integrated circuit containing CSEF circuits. A mismatch in the natural threshold of HCSEF and CSEF circuits, poor delay skew, and insufficient temperature and process compensation, all limit the functionality of HCSEF circuits (such as the exemplary HCSEF circuitry described in the referenced publication), when combined with CSEF circuits.
A further problem is that if the HCSEF are run with a lower rail voltage of -0.7 V, available for CSEF circuits, insufficient noise tolerance would result for the NOR.sub.i circuit.
Accordingly, it would be desirable to provide HCSEF circuitry that is compatible with CSEF circuits on a common integrated circuit. More particularly, it would be desirable to provide HCSEF circuitry that has a natural threshold equal to the threshold of any CSEF circuit to which it is coupled; HCSEF circuitry that has a delay skew of approximately 1:1 as compared with most HCSEF circuits which have delay skews in excess of 4:1; and HCSEF circuits which operate in combination with a bias generator to compensate for temperature and process variables.